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                <h2 id="1、前置知识"><a href="#1、前置知识" class="headerlink" title="1、前置知识"></a>1、前置知识</h2><p>HashMap 是我们常用的一个数据结构，它根据键的hashCode值存储数据，大多数情况下可以直接定位到它的值，因而具有很快的访问速度，但遍历顺序却不是确定的。HashMap 最多只允许一条记录的键为null，允许多条记录的值为null。HashMap 非线程安全，即任一时间可以有多个线程同时写HashMap，可能会导致数据的不一致。如果需要满足线程安全要求，可以使用Collections 的synchronizedMap 方法使HashMap具有线程安全的能力，或者使用 ConcurrentMap。不建议使用 Hashtable，它虽然能保证线程安全，但是内部都是使用了 Synchronized，并发性低。HashMap 的继承关系如下：</p>
<img src="https://cdn.jsdelivr.net/gh/kuangtf/PictureBed/img/image-20220327102116825.png" alt="image-20220327102116825" style="zoom:80%;" />

<h2 id="2、内部实现"><a href="#2、内部实现" class="headerlink" title="2、内部实现"></a>2、内部实现</h2><p>从结构上讲，HashMap 是数组+链表+红黑树（JDK1.8增加）实现的，如下图所示：</p>
<img src="https://cdn.jsdelivr.net/gh/kuangtf/PictureBed/img/image-20220327102128444.png" alt="image-20220327102128444" style="zoom:80%;" />

<p>这里需要明白两个问题：数据底层具体存储的是什么？这样的存储方式有什么优缺点？</p>
<p>我们先看看图中的这个Node源码是咋样的：</p>
<pre class="line-numbers language-java" data-language="java"><code class="language-java"><span class="token keyword">static</span> <span class="token keyword">class</span> <span class="token class-name">Node</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">K</span><span class="token punctuation">,</span><span class="token class-name">V</span><span class="token punctuation">></span></span> <span class="token keyword">implements</span> <span class="token class-name">Map<span class="token punctuation">.</span>Entry</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">K</span><span class="token punctuation">,</span><span class="token class-name">V</span><span class="token punctuation">></span></span> <span class="token punctuation">&#123;</span>
        <span class="token keyword">final</span> <span class="token keyword">int</span> hash<span class="token punctuation">;</span>    <span class="token comment">//用来定位数组索引位置</span>
        <span class="token keyword">final</span> <span class="token class-name">K</span> key<span class="token punctuation">;</span>
        <span class="token class-name">V</span> value<span class="token punctuation">;</span>
        <span class="token class-name">Node</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">K</span><span class="token punctuation">,</span><span class="token class-name">V</span><span class="token punctuation">></span></span> next<span class="token punctuation">;</span>   <span class="token comment">//链表的下一个node</span>

        <span class="token class-name">Node</span><span class="token punctuation">(</span><span class="token keyword">int</span> hash<span class="token punctuation">,</span> <span class="token class-name">K</span> key<span class="token punctuation">,</span> <span class="token class-name">V</span> value<span class="token punctuation">,</span> <span class="token class-name">Node</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">K</span><span class="token punctuation">,</span><span class="token class-name">V</span><span class="token punctuation">></span></span> next<span class="token punctuation">)</span> <span class="token punctuation">&#123;</span> <span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</span> <span class="token punctuation">&#125;</span>
        <span class="token keyword">public</span> <span class="token keyword">final</span> <span class="token class-name">K</span> <span class="token function">getKey</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">&#123;</span> <span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</span> <span class="token punctuation">&#125;</span>
        <span class="token keyword">public</span> <span class="token keyword">final</span> <span class="token class-name">V</span> <span class="token function">getValue</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">&#123;</span> <span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</span> <span class="token punctuation">&#125;</span>
        <span class="token keyword">public</span> <span class="token keyword">final</span> <span class="token class-name">String</span> <span class="token function">toString</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">&#123;</span> <span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</span> <span class="token punctuation">&#125;</span>
        <span class="token keyword">public</span> <span class="token keyword">final</span> <span class="token keyword">int</span> <span class="token function">hashCode</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">&#123;</span> <span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</span> <span class="token punctuation">&#125;</span>
        <span class="token keyword">public</span> <span class="token keyword">final</span> <span class="token class-name">V</span> <span class="token function">setValue</span><span class="token punctuation">(</span><span class="token class-name">V</span> newValue<span class="token punctuation">)</span> <span class="token punctuation">&#123;</span> <span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</span> <span class="token punctuation">&#125;</span>
        <span class="token keyword">public</span> <span class="token keyword">final</span> <span class="token keyword">boolean</span> <span class="token function">equals</span><span class="token punctuation">(</span><span class="token class-name">Object</span> o<span class="token punctuation">)</span> <span class="token punctuation">&#123;</span> <span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</span> <span class="token punctuation">&#125;</span>
<span class="token punctuation">&#125;</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>

<p>HashMap 中有一个非常重要的字段，就是 Node[] table，即哈希桶数组，Node 是 HashMap 的一个内部类，实现了Map.Entry 接口，本质就是键值对映射。</p>
<p>HashMap就是使用哈希表来存储的，哈希表为了解决冲突，可以采用开放地址法和链地址法，HashMap 中采用的就是链地址法。就是数组加链表的结合，每个数组元素上都是一个链表结构，当数据被Hash后，得到数组下标，把数据放到对应下标元素的链表上。比如执行下面的代码：</p>
<pre class="line-numbers language-java" data-language="java"><code class="language-java">map<span class="token punctuation">.</span><span class="token function">put</span><span class="token punctuation">(</span><span class="token string">"帅枫"</span><span class="token punctuation">,</span><span class="token string">"很帅"</span><span class="token punctuation">)</span><span class="token punctuation">;</span><span aria-hidden="true" class="line-numbers-rows"><span></span></span></code></pre>

<p>程序将调用hashCode()方法得到“帅枫”的hashCode值，再通过hash算法来定位该键值对的存储位置，有时两个key会定位到相同的位置，表明发生了哈希碰撞，当然Hash算法计算结果越分散，Hash碰撞的概率就越小，map的存取效率就越高。</p>
<p>如果哈希桶数组很大，即使较差的Hash算法也会比较分散，如果哈希桶数组数组很小，即使好的Hash算法也会出现较多碰撞，所以就需要在空间成本和时间成本之间权衡，其实就是在根据实际情况确定哈希桶数组的大小，并在此基础上设计好的hash算法减少Hash碰撞。那么通过什么方式来控制map使得Hash碰撞的概率又小，哈希桶数组（Node[] table）占用空间又少呢？答案就是好的Hash算法和扩容机制。</p>
<p>在理解Hash和扩容流程之前，我们得先了解下HashMap的几个字段。从HashMap的默认构造函数源码可知，构造函数就是对下面几个字段进行初始化，源码如下：</p>
<pre class="line-numbers language-java" data-language="java"><code class="language-java"><span class="token keyword">int</span> threshold<span class="token punctuation">;</span>             <span class="token comment">// 所能容纳的key-value对极限 </span>
<span class="token keyword">final</span> <span class="token keyword">float</span> loadFactor<span class="token punctuation">;</span>    <span class="token comment">// 负载因子</span>
<span class="token keyword">int</span> modCount<span class="token punctuation">;</span>  
<span class="token keyword">int</span> size<span class="token punctuation">;</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span></span></code></pre>

<p>首先，Node[] table的初始化长度length(默认值是16)，Load factor为负载因子(默认值是0.75)，threshold是HashMap所能容纳的最大数据量的Node(键值对)个数。threshold = length * Load factor。也就是说，在数组定义好长度之后，负载因子越大，所能容纳的键值对个数越多。</p>
<p>结合负载因子的定义公式可知，threshold就是在此Load factor和length(数组长度)对应下允许的最大元素数目，超过这个数目就重新resize(扩容)，扩容后的HashMap容量是之前容量的两倍。默认的负载因子0.75是对空间和时间效率的一个平衡选择，这个值可以大于1。</p>
<p>size这个字段其实很好理解，就是HashMap中实际存在的键值对数量，而modCount字段主要用来记录HashMap内部结构发生变化的次数，主要用于迭代的快速失败。</p>
<p>在HashMap中，哈希桶数组table的长度length大小必须为2的n次方(一定是合数)，HashMap采用这种非常规设计，主要是为了在取模和扩容时做优化，同时为了减少冲突，HashMap定位哈希桶索引位置时，也加入了高位参与运算的过程。</p>
<p>这里存在一个问题，即使负载因子和Hash算法设计的再合理，也免不了会出现拉链过长的情况，一旦出现拉链过长，则会严重影响HashMap的性能。于是，在JDK1.8版本中，对数据结构做了进一步的优化，引入了红黑树。而当链表长度太长（默认超过8）时，链表就转换为红黑树，利用红黑树快速增删改查的特点提高HashMap的性能，将链表转换成红黑树前会判断，如果当前数组的长度小于 64，那么会选择先进行数组扩容，而不是转换为红黑树。</p>
<h2 id="3、方法的实现"><a href="#3、方法的实现" class="headerlink" title="3、方法的实现"></a>3、方法的实现</h2><h3 id="1、确定哈希桶数组索引位置"><a href="#1、确定哈希桶数组索引位置" class="headerlink" title="1、确定哈希桶数组索引位置"></a>1、确定哈希桶数组索引位置</h3><p>不管增加、删除、查找键值对，定位到哈希桶数组的位置都是很关键的第一步。前面说过HashMap的数据结构是数组和链表的结合，所以我们当然希望这个HashMap里面的元素位置尽量分布均匀些，尽量使得每个位置上的元素数量只有一个，那么当我们用hash算法求得这个位置的时候，马上就可以知道对应位置的元素就是我们要的，不用遍历链表，大大优化了查询的效率。HashMap定位数组索引位置，直接决定了hash方法的离散性能，下面看一下JDK7和JDK8中hash方法的实现：</p>
<p>JDK8：</p>
<pre class="line-numbers language-java" data-language="java"><code class="language-java"><span class="token keyword">static</span> <span class="token keyword">final</span> <span class="token keyword">int</span> <span class="token function">hash</span><span class="token punctuation">(</span><span class="token class-name">Object</span> key<span class="token punctuation">)</span> <span class="token punctuation">&#123;</span>
    <span class="token keyword">int</span> h<span class="token punctuation">;</span>
    <span class="token comment">// key.hashCode()：返回散列值也就是hashcode</span>
    <span class="token comment">// ^ ：按位异或</span>
    <span class="token comment">// >>>:无符号右移，忽略符号位，空位都以0补齐</span>
    <span class="token keyword">return</span> <span class="token punctuation">(</span>key <span class="token operator">==</span> <span class="token keyword">null</span><span class="token punctuation">)</span> <span class="token operator">?</span> <span class="token number">0</span> <span class="token operator">:</span> <span class="token punctuation">(</span>h <span class="token operator">=</span> key<span class="token punctuation">.</span><span class="token function">hashCode</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token operator">^</span> <span class="token punctuation">(</span>h <span class="token operator">>>></span> <span class="token number">16</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">&#125;</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>

<p>JDK7：</p>
<pre class="line-numbers language-java" data-language="java"><code class="language-java"><span class="token keyword">static</span> <span class="token keyword">int</span> <span class="token function">hash</span><span class="token punctuation">(</span><span class="token keyword">int</span> h<span class="token punctuation">)</span> <span class="token punctuation">&#123;</span>
    h <span class="token operator">^=</span> <span class="token punctuation">(</span>h <span class="token operator">>>></span> <span class="token number">20</span><span class="token punctuation">)</span> <span class="token operator">^</span> <span class="token punctuation">(</span>h <span class="token operator">>>></span> <span class="token number">12</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token keyword">return</span> h <span class="token operator">^</span> <span class="token punctuation">(</span>h <span class="token operator">>>></span> <span class="token number">7</span><span class="token punctuation">)</span> <span class="token operator">^</span> <span class="token punctuation">(</span>h <span class="token operator">>>></span> <span class="token number">4</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">&#125;</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span></span></code></pre>

<p>上面的方法算出了key 的哈希值，我们首先想到的就是把hash值对数组长度取模运算，这样一来，元素的分布相对来说是比较均匀的。但是，模运算的消耗还是比较大的，在HashMap中是这样做的：</p>
<pre class="line-numbers language-java" data-language="java"><code class="language-java"><span class="token keyword">static</span> <span class="token keyword">int</span> <span class="token function">indexFor</span><span class="token punctuation">(</span><span class="token keyword">int</span> h<span class="token punctuation">,</span> <span class="token keyword">int</span> length<span class="token punctuation">)</span> <span class="token punctuation">&#123;</span>  
	<span class="token comment">//jdk1.7的源码，jdk1.8没有这个方法，但是实现原理一样的</span>
    <span class="token keyword">return</span> h <span class="token operator">&amp;</span> <span class="token punctuation">(</span>length<span class="token operator">-</span><span class="token number">1</span><span class="token punctuation">)</span><span class="token punctuation">;</span>  <span class="token comment">//第三步 取模运算</span>
<span class="token punctuation">&#125;</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span></span></code></pre>

<p>这个方法非常巧妙，它通过h &amp; (table.length -1)来得到该对象的保存位，而HashMap底层数组的长度总是2的n次方，这是HashMap在速度上的优化。当length总是2的n次方时，h&amp; (length-1)运算等价于对length取模，也就是h%length，但是&amp;比%具有更高的效率。</p>
<p>在JDK1.8的实现中，优化了高位运算的算法，通过hashCode()的高16位异或低16位实现的：(h = k.hashCode()) ^ (h &gt;&gt;&gt; 16)，主要是从速度、功效、质量来考虑的，这么做可以在数组table的length比较小的时候，也能保证考虑到高低Bit都参与到Hash的计算中，同时不会有太大的开销。</p>
<h3 id="2、HashMap-的-put-方法"><a href="#2、HashMap-的-put-方法" class="headerlink" title="2、HashMap 的 put 方法"></a>2、HashMap 的 put 方法</h3><p>HashMap 的 put 方法主要是下面的这个流程：JDK8版本</p>
<img src="https://cdn.jsdelivr.net/gh/kuangtf/PictureBed/img/image-20220327102139758.png" alt="image-20220327102139758" style="zoom:80%;" />

<p>简单解释一下流程：</p>
<ol>
<li>判断键值对数组table[i]是否为空或为null，否则执行resize()进行扩容。</li>
<li>根据键值key计算hash值得到插入的数组索引i，如果table[i]==null，直接新建节点添加，转向⑥，如果table[i]不为空，转向③。</li>
<li>判断table[i]这槽位上的所有的元素是否有和key一样，如果相同直接覆盖value，否则转向④。</li>
<li>判断table[i] 是否为treeNode，即table[i] 是否是红黑树，如果是红黑树，则直接在树中插入键值对，转向⑥，否则转向⑤。</li>
<li>遍历table[i]，判断链表长度是否大于8并且数组长度超过64，如果是的话则把链表转换为红黑树，在红黑树中执行插入操作，转向⑥。否则再进行判断链表长度是否小于64并且链表长度大于8，如果是的话就进行扩容之后再 rehash，否则进行链表的插入操作，转向⑥。</li>
<li>插入成功后，判断实际存在的键值对数量size是否超多了最大容量threshold，如果超过，进行扩容。</li>
</ol>
<p>源码如下：JDK8版本</p>
<pre class="line-numbers language-java" data-language="java"><code class="language-java"><span class="token keyword">public</span> <span class="token class-name">V</span> <span class="token function">put</span><span class="token punctuation">(</span><span class="token class-name">K</span> key<span class="token punctuation">,</span> <span class="token class-name">V</span> value<span class="token punctuation">)</span> <span class="token punctuation">&#123;</span>
    <span class="token keyword">return</span> <span class="token function">putVal</span><span class="token punctuation">(</span><span class="token function">hash</span><span class="token punctuation">(</span>key<span class="token punctuation">)</span><span class="token punctuation">,</span> key<span class="token punctuation">,</span> value<span class="token punctuation">,</span> <span class="token boolean">false</span><span class="token punctuation">,</span> <span class="token boolean">true</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">&#125;</span>

<span class="token keyword">final</span> <span class="token class-name">V</span> <span class="token function">putVal</span><span class="token punctuation">(</span><span class="token keyword">int</span> hash<span class="token punctuation">,</span> <span class="token class-name">K</span> key<span class="token punctuation">,</span> <span class="token class-name">V</span> value<span class="token punctuation">,</span> <span class="token keyword">boolean</span> onlyIfAbsent<span class="token punctuation">,</span> <span class="token keyword">boolean</span> evict<span class="token punctuation">)</span> <span class="token punctuation">&#123;</span>
    <span class="token class-name">Node</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">K</span><span class="token punctuation">,</span><span class="token class-name">V</span><span class="token punctuation">></span></span><span class="token punctuation">[</span><span class="token punctuation">]</span> tab<span class="token punctuation">;</span> <span class="token class-name">Node</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">K</span><span class="token punctuation">,</span><span class="token class-name">V</span><span class="token punctuation">></span></span> p<span class="token punctuation">;</span> <span class="token keyword">int</span> n<span class="token punctuation">,</span> i<span class="token punctuation">;</span>
    <span class="token comment">// table未初始化或者长度为0，进行扩容</span>
    <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token punctuation">(</span>tab <span class="token operator">=</span> table<span class="token punctuation">)</span> <span class="token operator">==</span> <span class="token keyword">null</span> <span class="token operator">||</span> <span class="token punctuation">(</span>n <span class="token operator">=</span> tab<span class="token punctuation">.</span>length<span class="token punctuation">)</span> <span class="token operator">==</span> <span class="token number">0</span><span class="token punctuation">)</span> n <span class="token operator">=</span> <span class="token punctuation">(</span>tab <span class="token operator">=</span> <span class="token function">resize</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">.</span>length<span class="token punctuation">;</span>
    <span class="token comment">// (n - 1) &amp; hash 确定元素存放在哪个桶中，桶为空，新生成结点放入桶中(此时，这个结点是放在数组中)</span>
    <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token punctuation">(</span>p <span class="token operator">=</span> tab<span class="token punctuation">[</span>i <span class="token operator">=</span> <span class="token punctuation">(</span>n <span class="token operator">-</span> <span class="token number">1</span><span class="token punctuation">)</span> <span class="token operator">&amp;</span> hash<span class="token punctuation">]</span><span class="token punctuation">)</span> <span class="token operator">==</span> <span class="token keyword">null</span><span class="token punctuation">)</span> tab<span class="token punctuation">[</span>i<span class="token punctuation">]</span> <span class="token operator">=</span> <span class="token function">newNode</span><span class="token punctuation">(</span>hash<span class="token punctuation">,</span> key<span class="token punctuation">,</span> value<span class="token punctuation">,</span> <span class="token keyword">null</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token comment">// 桶中已经存在元素</span>
    <span class="token keyword">else</span> <span class="token punctuation">&#123;</span>
        <span class="token class-name">Node</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">K</span><span class="token punctuation">,</span><span class="token class-name">V</span><span class="token punctuation">></span></span> e<span class="token punctuation">;</span> <span class="token class-name">K</span> k<span class="token punctuation">;</span>
        <span class="token comment">// 比较桶中第一个元素(数组中的结点)的hash值相等，key相等</span>
        <span class="token keyword">if</span> <span class="token punctuation">(</span>p<span class="token punctuation">.</span>hash <span class="token operator">==</span> hash <span class="token operator">&amp;&amp;</span> <span class="token punctuation">(</span><span class="token punctuation">(</span>k <span class="token operator">=</span> p<span class="token punctuation">.</span>key<span class="token punctuation">)</span> <span class="token operator">==</span> key <span class="token operator">||</span> <span class="token punctuation">(</span>key <span class="token operator">!=</span> <span class="token keyword">null</span> <span class="token operator">&amp;&amp;</span> key<span class="token punctuation">.</span><span class="token function">equals</span><span class="token punctuation">(</span>k<span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">)</span>
            <span class="token comment">// 将第一个元素赋值给e，用e来记录</span>
            e <span class="token operator">=</span> p<span class="token punctuation">;</span>
        <span class="token comment">// hash值不相等，即key不相等；为红黑树结点</span>
        <span class="token keyword">else</span> <span class="token keyword">if</span> <span class="token punctuation">(</span>p <span class="token keyword">instanceof</span> <span class="token class-name">TreeNode</span><span class="token punctuation">)</span>
            <span class="token comment">// 放入树中</span>
            e <span class="token operator">=</span> <span class="token punctuation">(</span><span class="token punctuation">(</span><span class="token class-name">TreeNode</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">K</span><span class="token punctuation">,</span><span class="token class-name">V</span><span class="token punctuation">></span></span><span class="token punctuation">)</span>p<span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">putTreeVal</span><span class="token punctuation">(</span><span class="token keyword">this</span><span class="token punctuation">,</span> tab<span class="token punctuation">,</span> hash<span class="token punctuation">,</span> key<span class="token punctuation">,</span> value<span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token comment">// 为链表结点</span>
        <span class="token keyword">else</span> <span class="token punctuation">&#123;</span>
            <span class="token comment">// 在链表最末插入结点</span>
            <span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">int</span> binCount <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span> <span class="token punctuation">;</span> <span class="token operator">++</span>binCount<span class="token punctuation">)</span> <span class="token punctuation">&#123;</span>
                <span class="token comment">// 到达链表的尾部</span>
                <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token punctuation">(</span>e <span class="token operator">=</span> p<span class="token punctuation">.</span>next<span class="token punctuation">)</span> <span class="token operator">==</span> <span class="token keyword">null</span><span class="token punctuation">)</span> <span class="token punctuation">&#123;</span>
                    <span class="token comment">// 在尾部插入新结点</span>
                    p<span class="token punctuation">.</span>next <span class="token operator">=</span> <span class="token function">newNode</span><span class="token punctuation">(</span>hash<span class="token punctuation">,</span> key<span class="token punctuation">,</span> value<span class="token punctuation">,</span> <span class="token keyword">null</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
                    <span class="token comment">// 结点数量达到阈值(默认为 8 )，执行 treeifyBin 方法</span>
                    <span class="token comment">// 这个方法会根据 HashMap 数组来决定是否转换为红黑树。</span>
                    <span class="token comment">// 只有当数组长度大于或者等于 64 的情况下，才会执行转换红黑树操作，以减少搜索时间。否则，就是只是对数组扩容。</span>
                    <span class="token keyword">if</span> <span class="token punctuation">(</span>binCount <span class="token operator">>=</span> TREEIFY_THRESHOLD <span class="token operator">-</span> <span class="token number">1</span><span class="token punctuation">)</span> <span class="token comment">// -1 for 1st</span>
                        <span class="token function">treeifyBin</span><span class="token punctuation">(</span>tab<span class="token punctuation">,</span> hash<span class="token punctuation">)</span><span class="token punctuation">;</span>
                    <span class="token comment">// 跳出循环</span>
                    <span class="token keyword">break</span><span class="token punctuation">;</span>
                <span class="token punctuation">&#125;</span>
                <span class="token comment">// 判断链表中结点的key值与插入的元素的key值是否相等</span>
                <span class="token keyword">if</span> <span class="token punctuation">(</span>e<span class="token punctuation">.</span>hash <span class="token operator">==</span> hash <span class="token operator">&amp;&amp;</span> <span class="token punctuation">(</span><span class="token punctuation">(</span>k <span class="token operator">=</span> e<span class="token punctuation">.</span>key<span class="token punctuation">)</span> <span class="token operator">==</span> key <span class="token operator">||</span> <span class="token punctuation">(</span>key <span class="token operator">!=</span> <span class="token keyword">null</span> <span class="token operator">&amp;&amp;</span> key<span class="token punctuation">.</span><span class="token function">equals</span><span class="token punctuation">(</span>k<span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">)</span>
                    <span class="token comment">// 相等，跳出循环</span>
                    <span class="token keyword">break</span><span class="token punctuation">;</span>
                <span class="token comment">// 用于遍历桶中的链表，与前面的e = p.next组合，可以遍历链表</span>
                p <span class="token operator">=</span> e<span class="token punctuation">;</span>
            <span class="token punctuation">&#125;</span>
        <span class="token punctuation">&#125;</span>
        <span class="token comment">// 表示在桶中找到key值、hash值与插入元素相等的结点</span>
        <span class="token keyword">if</span> <span class="token punctuation">(</span>e <span class="token operator">!=</span> <span class="token keyword">null</span><span class="token punctuation">)</span> <span class="token punctuation">&#123;</span>
            <span class="token comment">// 记录e的value</span>
            <span class="token class-name">V</span> oldValue <span class="token operator">=</span> e<span class="token punctuation">.</span>value<span class="token punctuation">;</span>
            <span class="token comment">// onlyIfAbsent为false或者旧值为null</span>
            <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token operator">!</span>onlyIfAbsent <span class="token operator">||</span> oldValue <span class="token operator">==</span> <span class="token keyword">null</span><span class="token punctuation">)</span>
                <span class="token comment">//用新值替换旧值</span>
                e<span class="token punctuation">.</span>value <span class="token operator">=</span> value<span class="token punctuation">;</span>
            <span class="token comment">// 访问后回调</span>
            <span class="token function">afterNodeAccess</span><span class="token punctuation">(</span>e<span class="token punctuation">)</span><span class="token punctuation">;</span>
            <span class="token comment">// 返回旧值</span>
            <span class="token keyword">return</span> oldValue<span class="token punctuation">;</span>
        <span class="token punctuation">&#125;</span>
    <span class="token punctuation">&#125;</span>
    <span class="token comment">// 结构性修改</span>
    <span class="token operator">++</span>modCount<span class="token punctuation">;</span>
    <span class="token comment">// 实际大小大于阈值则扩容</span>
    <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token operator">++</span>size <span class="token operator">></span> threshold<span class="token punctuation">)</span>
        <span class="token function">resize</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token comment">// 插入后回调</span>
    <span class="token function">afterNodeInsertion</span><span class="token punctuation">(</span>evict<span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token keyword">return</span> <span class="token keyword">null</span><span class="token punctuation">;</span>
<span class="token punctuation">&#125;</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>

<p>JDK7与JDK8有点区别，感兴趣的同学可以自己去看看，不要看这个步骤很多，梳理清楚了还是很容易记住的。</p>
<blockquote>
<p>JDK8是链表的时候采用的是尾插法，JDK7中采用的是头插法。</p>
</blockquote>
<h3 id="3、HashMap-的-get-方法"><a href="#3、HashMap-的-get-方法" class="headerlink" title="3、HashMap 的 get 方法"></a>3、HashMap 的 get 方法</h3><p>get 方法没有 put 方法那么多判断，就是遍历，下面是源码：</p>
<pre class="line-numbers language-java" data-language="java"><code class="language-java"><span class="token keyword">public</span> <span class="token class-name">V</span> <span class="token function">get</span><span class="token punctuation">(</span><span class="token class-name">Object</span> key<span class="token punctuation">)</span> <span class="token punctuation">&#123;</span>
    <span class="token class-name">Node</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">K</span><span class="token punctuation">,</span><span class="token class-name">V</span><span class="token punctuation">></span></span> e<span class="token punctuation">;</span>
    <span class="token keyword">return</span> <span class="token punctuation">(</span>e <span class="token operator">=</span> <span class="token function">getNode</span><span class="token punctuation">(</span><span class="token function">hash</span><span class="token punctuation">(</span>key<span class="token punctuation">)</span><span class="token punctuation">,</span> key<span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token operator">==</span> <span class="token keyword">null</span> <span class="token operator">?</span> <span class="token keyword">null</span> <span class="token operator">:</span> e<span class="token punctuation">.</span>value<span class="token punctuation">;</span>
<span class="token punctuation">&#125;</span>

<span class="token keyword">final</span> <span class="token class-name">Node</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">K</span><span class="token punctuation">,</span><span class="token class-name">V</span><span class="token punctuation">></span></span> <span class="token function">getNode</span><span class="token punctuation">(</span><span class="token keyword">int</span> hash<span class="token punctuation">,</span> <span class="token class-name">Object</span> key<span class="token punctuation">)</span> <span class="token punctuation">&#123;</span>
    <span class="token class-name">Node</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">K</span><span class="token punctuation">,</span><span class="token class-name">V</span><span class="token punctuation">></span></span><span class="token punctuation">[</span><span class="token punctuation">]</span> tab<span class="token punctuation">;</span> <span class="token class-name">Node</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">K</span><span class="token punctuation">,</span><span class="token class-name">V</span><span class="token punctuation">></span></span> first<span class="token punctuation">,</span> e<span class="token punctuation">;</span> <span class="token keyword">int</span> n<span class="token punctuation">;</span> <span class="token class-name">K</span> k<span class="token punctuation">;</span>
    <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token punctuation">(</span>tab <span class="token operator">=</span> table<span class="token punctuation">)</span> <span class="token operator">!=</span> <span class="token keyword">null</span> <span class="token operator">&amp;&amp;</span> <span class="token punctuation">(</span>n <span class="token operator">=</span> tab<span class="token punctuation">.</span>length<span class="token punctuation">)</span> <span class="token operator">></span> <span class="token number">0</span> <span class="token operator">&amp;&amp;</span>
        <span class="token punctuation">(</span>first <span class="token operator">=</span> tab<span class="token punctuation">[</span><span class="token punctuation">(</span>n <span class="token operator">-</span> <span class="token number">1</span><span class="token punctuation">)</span> <span class="token operator">&amp;</span> hash<span class="token punctuation">]</span><span class="token punctuation">)</span> <span class="token operator">!=</span> <span class="token keyword">null</span><span class="token punctuation">)</span> <span class="token punctuation">&#123;</span>
        <span class="token comment">// 数组元素相等</span>
        <span class="token keyword">if</span> <span class="token punctuation">(</span>first<span class="token punctuation">.</span>hash <span class="token operator">==</span> hash <span class="token operator">&amp;&amp;</span> <span class="token comment">// always check first node</span>
            <span class="token punctuation">(</span><span class="token punctuation">(</span>k <span class="token operator">=</span> first<span class="token punctuation">.</span>key<span class="token punctuation">)</span> <span class="token operator">==</span> key <span class="token operator">||</span> <span class="token punctuation">(</span>key <span class="token operator">!=</span> <span class="token keyword">null</span> <span class="token operator">&amp;&amp;</span> key<span class="token punctuation">.</span><span class="token function">equals</span><span class="token punctuation">(</span>k<span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">)</span>
            <span class="token keyword">return</span> first<span class="token punctuation">;</span>
        <span class="token comment">// 桶中不止一个节点</span>
        <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token punctuation">(</span>e <span class="token operator">=</span> first<span class="token punctuation">.</span>next<span class="token punctuation">)</span> <span class="token operator">!=</span> <span class="token keyword">null</span><span class="token punctuation">)</span> <span class="token punctuation">&#123;</span>
            <span class="token comment">// 在树中get</span>
            <span class="token keyword">if</span> <span class="token punctuation">(</span>first <span class="token keyword">instanceof</span> <span class="token class-name">TreeNode</span><span class="token punctuation">)</span>
                <span class="token keyword">return</span> <span class="token punctuation">(</span><span class="token punctuation">(</span><span class="token class-name">TreeNode</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">K</span><span class="token punctuation">,</span><span class="token class-name">V</span><span class="token punctuation">></span></span><span class="token punctuation">)</span>first<span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">getTreeNode</span><span class="token punctuation">(</span>hash<span class="token punctuation">,</span> key<span class="token punctuation">)</span><span class="token punctuation">;</span>
            <span class="token comment">// 在链表中get</span>
            <span class="token keyword">do</span> <span class="token punctuation">&#123;</span>
                <span class="token keyword">if</span> <span class="token punctuation">(</span>e<span class="token punctuation">.</span>hash <span class="token operator">==</span> hash <span class="token operator">&amp;&amp;</span>
                    <span class="token punctuation">(</span><span class="token punctuation">(</span>k <span class="token operator">=</span> e<span class="token punctuation">.</span>key<span class="token punctuation">)</span> <span class="token operator">==</span> key <span class="token operator">||</span> <span class="token punctuation">(</span>key <span class="token operator">!=</span> <span class="token keyword">null</span> <span class="token operator">&amp;&amp;</span> key<span class="token punctuation">.</span><span class="token function">equals</span><span class="token punctuation">(</span>k<span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">)</span>
                    <span class="token keyword">return</span> e<span class="token punctuation">;</span>
            <span class="token punctuation">&#125;</span> <span class="token keyword">while</span> <span class="token punctuation">(</span><span class="token punctuation">(</span>e <span class="token operator">=</span> e<span class="token punctuation">.</span>next<span class="token punctuation">)</span> <span class="token operator">!=</span> <span class="token keyword">null</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token punctuation">&#125;</span>
    <span class="token punctuation">&#125;</span>
    <span class="token keyword">return</span> <span class="token keyword">null</span><span class="token punctuation">;</span>
<span class="token punctuation">&#125;</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>

<h3 id="4、HashMap-中的-resize-方法"><a href="#4、HashMap-中的-resize-方法" class="headerlink" title="4、HashMap 中的 resize 方法"></a>4、HashMap 中的 resize 方法</h3><p>扩容(resize)就是重新计算容量，向HashMap对象里不停的添加元素，而HashMap对象内部的数组无法装载更多的元素时，对象就需要扩大数组的长度，以便能装入更多的元素。当然Java里的数组是无法自动扩容的，方法是使用一个新的数组代替已有的容量小的数组。</p>
<p>分析下resize的源码，鉴于JDK1.8融入了红黑树，较复杂，为了便于理解我们仍然使用JDK1.7的代码，好理解一些，本质上区别不大。</p>
<pre class="line-numbers language-java" data-language="java"><code class="language-java"><span class="token keyword">void</span> <span class="token function">resize</span><span class="token punctuation">(</span><span class="token keyword">int</span> newCapacity<span class="token punctuation">)</span> <span class="token punctuation">&#123;</span> 
    <span class="token comment">//传入新的容量    </span>
    <span class="token class-name">Entry</span><span class="token punctuation">[</span><span class="token punctuation">]</span> oldTable <span class="token operator">=</span> table<span class="token punctuation">;</span>   
    <span class="token comment">//引用扩容前的Entry数组    </span>
    <span class="token keyword">int</span> oldCapacity <span class="token operator">=</span> oldTable<span class="token punctuation">.</span>length<span class="token punctuation">;</span> 
    <span class="token keyword">if</span> <span class="token punctuation">(</span>oldCapacity <span class="token operator">==</span> MAXIMUM_CAPACITY<span class="token punctuation">)</span> <span class="token punctuation">&#123;</span>
        <span class="token comment">//扩容前的数组大小如果已经达到最大(2^30)了     </span>
        threshold <span class="token operator">=</span> <span class="token class-name">Integer</span><span class="token punctuation">.</span>MAX_VALUE<span class="token punctuation">;</span> 
        <span class="token comment">//修改阈值为int的最大值(2^31-1)，这样以后就不会扩容了 </span>
        <span class="token keyword">return</span><span class="token punctuation">;</span> 
    <span class="token punctuation">&#125;</span>     
    <span class="token comment">//初始化一个新的Entry数组 </span>
    <span class="token class-name">Entry</span><span class="token punctuation">[</span><span class="token punctuation">]</span> newTable <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Entry</span><span class="token punctuation">[</span>newCapacity<span class="token punctuation">]</span><span class="token punctuation">;</span>  
    <span class="token comment">//将数据转移到新的Entry数组里  </span>
    <span class="token function">transfer</span><span class="token punctuation">(</span>newTable<span class="token punctuation">)</span><span class="token punctuation">;</span>              
    <span class="token comment">//HashMap的table属性引用新的Entry数组  </span>
    table <span class="token operator">=</span> newTable<span class="token punctuation">;</span>                 
    <span class="token comment">//修改阈值</span>
    threshold <span class="token operator">=</span> <span class="token punctuation">(</span><span class="token keyword">int</span><span class="token punctuation">)</span><span class="token punctuation">(</span>newCapacity <span class="token operator">*</span> loadFactor<span class="token punctuation">)</span><span class="token punctuation">;</span>  
<span class="token punctuation">&#125;</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>

<p>这里就是使用一个容量更大的数组来代替已有的容量小的数组，transfer()方法将原有Entry数组的元素拷贝到新的Entry数组里。</p>
<pre class="line-numbers language-java" data-language="java"><code class="language-java"><span class="token keyword">void</span> <span class="token function">transfer</span><span class="token punctuation">(</span><span class="token class-name">Entry</span><span class="token punctuation">[</span><span class="token punctuation">]</span> newTable<span class="token punctuation">)</span> <span class="token punctuation">&#123;</span>     
    <span class="token class-name">Entry</span><span class="token punctuation">[</span><span class="token punctuation">]</span> src <span class="token operator">=</span> table<span class="token punctuation">;</span>  
    <span class="token comment">//src引用了旧的Entry数组  </span>
    <span class="token keyword">int</span> newCapacity <span class="token operator">=</span> newTable<span class="token punctuation">.</span>length<span class="token punctuation">;</span>   
    <span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">int</span> j <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span> j <span class="token operator">&lt;</span> src<span class="token punctuation">.</span>length<span class="token punctuation">;</span> j<span class="token operator">++</span><span class="token punctuation">)</span> <span class="token punctuation">&#123;</span>
        <span class="token comment">//遍历旧的Entry数组       </span>
        <span class="token class-name">Entry</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">K</span><span class="token punctuation">,</span><span class="token class-name">V</span><span class="token punctuation">></span></span> e <span class="token operator">=</span> src<span class="token punctuation">[</span>j<span class="token punctuation">]</span><span class="token punctuation">;</span>    
        <span class="token comment">//取得旧Entry数组的每个元素      </span>
        <span class="token keyword">if</span> <span class="token punctuation">(</span>e <span class="token operator">!=</span> <span class="token keyword">null</span><span class="token punctuation">)</span> <span class="token punctuation">&#123;</span>       
            src<span class="token punctuation">[</span>j<span class="token punctuation">]</span> <span class="token operator">=</span> <span class="token keyword">null</span><span class="token punctuation">;</span>
            <span class="token comment">//释放旧Entry数组的对象引用（for循环后，旧的Entry数组不再引用任何对象）       </span>
            <span class="token keyword">do</span> <span class="token punctuation">&#123;</span>         
                <span class="token class-name">Entry</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">K</span><span class="token punctuation">,</span><span class="token class-name">V</span><span class="token punctuation">></span></span> next <span class="token operator">=</span> e<span class="token punctuation">.</span>next<span class="token punctuation">;</span>   
                <span class="token keyword">int</span> i <span class="token operator">=</span> <span class="token function">indexFor</span><span class="token punctuation">(</span>e<span class="token punctuation">.</span>hash<span class="token punctuation">,</span> newCapacity<span class="token punctuation">)</span><span class="token punctuation">;</span> 
                <span class="token comment">//重新计算每个元素在数组中的位置    </span>
                e<span class="token punctuation">.</span>next <span class="token operator">=</span> newTable<span class="token punctuation">[</span>i<span class="token punctuation">]</span><span class="token punctuation">;</span> 
                <span class="token comment">//标记[1]          </span>
                newTable<span class="token punctuation">[</span>i<span class="token punctuation">]</span> <span class="token operator">=</span> e<span class="token punctuation">;</span>  
                <span class="token comment">//将元素放在数组上   </span>
                e <span class="token operator">=</span> next<span class="token punctuation">;</span>          
                <span class="token comment">//访问下一个Entry链上的元素     </span>
            <span class="token punctuation">&#125;</span> <span class="token keyword">while</span> <span class="token punctuation">(</span>e <span class="token operator">!=</span> <span class="token keyword">null</span><span class="token punctuation">)</span><span class="token punctuation">;</span>      
        <span class="token punctuation">&#125;</span>   
    <span class="token punctuation">&#125;</span>
<span class="token punctuation">&#125;</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>

<p>newTable[i]的引用赋给了e.next，也就是使用了单链表的头插入方式，同一位置上新元素总会被放在链表的头部位置；这样先放在一个索引上的元素终会被放到Entry链的尾部(如果发生了hash冲突的话）。这是JDK7的版本。</p>
<p>那 JDK 8有哪些不同呢？</p>
<p>我们使用的是2次幂的扩展(指长度扩为原来2倍)，所以，元素的位置要么是在原位置，要么是在原位置再移动2次幂的位置。不需要像JDK1.7的实现那样重新计算hash，只需要看看原来的hash值新增的那个bit是1还是0就好了，是0的话索引没变，是1的话索引变成“原索引+原数组长度”。这个设计确实非常的巧妙，既省去了重新计算hash值的时间，而且同时，由于新增的1bit是0还是1可以认为是随机的，因此resize的过程，均匀的把之前的冲突的节点分散到新的bucket了。</p>
<blockquote>
<p>注意，JDK1.7中rehash的时候，旧链表迁移新链表的时候，如果在新表的数组索引位置相同，则链表元素会倒置，JDK1.8不会倒置。</p>
</blockquote>
<h2 id="4、HashMap-的线程安全性"><a href="#4、HashMap-的线程安全性" class="headerlink" title="4、HashMap 的线程安全性"></a>4、HashMap 的线程安全性</h2><p>再多线程使用场景中，应该尽量避免使用线程不安全的HashMap，而使用线程安全的ConcurrentHashMap。那么为什么说HashMap是线程不安全的呢？</p>
<ul>
<li>JDK1.7 中，由于多线程对HashMap进行扩容，调用了HashMap的transfer()，具体原因：某个线程执行过程中，被挂起，其他线程已经完成数据迁移，等CPU资源释放后被挂起的线程重新执行之前的逻辑，数据已经被改变，造成死循环、数据丢失。</li>
<li>JDK1.8 中，由于多线程对HashMap进行put操作，调用了HashMap的putVal()，具体原因：假设两个线程A、B都在进行put操作，并且hash函数计算出的插入下标是相同的，当线程A在判断了hash的碰撞之后，插入之前，由于时间片耗尽导致被挂起，而线程B得到时间片后在该下标处插入了元素，完成了正常的插入，然后线程A获得时间片，由于之前已经进行了hash碰撞的判断，所有此时不会再进行判断，而是直接进行插入，这就导致了线程B插入的数据被线程A覆盖了，从而线程不安全。</li>
</ul>
<p>具体的原因大家可以自己去看看源码，一切答案都在源码之中。</p>
<blockquote>
<p>巨人的肩膀：</p>
<p><a target="_blank" rel="noopener" href="https://tech.meituan.com/2016/06/24/java-hashmap.html">https://tech.meituan.com/2016/06/24/java-hashmap.html</a></p>
<p><a target="_blank" rel="noopener" href="https://snailclimb.gitee.io/javaguide/#/docs/java/collection/HashMap(JDK1.8)%E6%BA%90%E7%A0%81+%E5%BA%95%E5%B1%82%E6%95%B0%E6%8D%AE%E7%BB%93%E6%9E%84%E5%88%86%E6%9E%90">https://snailclimb.gitee.io/javaguide/#/docs/java/collection/HashMap(JDK1.8)%E6%BA%90%E7%A0%81+%E5%BA%95%E5%B1%82%E6%95%B0%E6%8D%AE%E7%BB%93%E6%9E%84%E5%88%86%E6%9E%90</a></p>
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